It is usually impossible to add new syntax to Perl without breaking some existing programs. This pragma provides a way to minimize that risk. New syntactic constructs, or new semantic meanings to older constructs, can be enabled by use feature 'foo', and will be parsed only when the appropriate feature pragma is in scope. (Nevertheless, the CORE:: prefix provides access to all Perl keywords, regardless of this pragma.)

use feature 'unicode_strings' tells the compiler to use Unicode rules in all string operations executed within its scope (unless they are also within the scope of either use locale or use bytes). The same applies to all regular expressions compiled within the scope, even if executed outside it. It does not change the internal representation of strings, but only how they are interpreted.

no feature 'unicode_strings' tells the compiler to use the traditional Perl rules wherein the native character set rules is used unless it is clear to Perl that Unicode is desired. This can lead to some surprises when the behavior suddenly changes. (See "The "Unicode Bug"" in perlunicode for details.) For this reason, if you are potentially using Unicode in your program, the use feature 'unicode_strings' subpragma is strongly recommended.

Under the unicode_eval feature, Perl's eval function, when passed a string, will evaluate it as a string of characters, ignoring any use utf8 declarations. use utf8 exists to declare the encoding of the script, which only makes sense for a stream of bytes, not a string of characters. Source filters are forbidden, as they also really only make sense on strings of bytes. Any attempt to activate a source filter will result in an error.

The evalbytes feature enables the evalbytes keyword, which evaluates the argument passed to it as a string of bytes. It dies if the string contains any characters outside the 8-bit range. Source filters work within evalbytes: they apply to the contents of the string being evaluated.

WARNING: This feature is still experimental and the implementation may change in future versions of Perl. For this reason, Perl will warn when you use the feature, unless you have explicitly disabled the warning:

WARNING: This feature is still experimental and the implementation may change in future versions of Perl. For this reason, Perl will warn when you use the feature, unless you have explicitly disabled the warning:

WARNING: This feature is still experimental and the implementation may change in future versions of Perl. For this reason, Perl will warn when you use the feature, unless you have explicitly disabled the warning:

WARNING: This feature is still experimental and the implementation may change in future versions of Perl. For this reason, Perl will warn when you use the feature, unless you have explicitly disabled the warning:

WARNING: This feature is still experimental and the implementation may change in future versions of Perl. For this reason, Perl will warn when you use the feature, unless you have explicitly disabled the warning: